In many machines, crankshafts serve the important function of converting rectilinear motion to rotary motion. For example, in machines including internal combustion engines with pistons, such as automobiles and airplanes, a crankshaft is used to convert the rectilinear movement of the pistons created upon combustion into the rotary motion ultimately used to drive the wheels, propeller, or the like. As used in these types of arrangements, the crankshaft typically includes main bearing journals and connecting rod journals, both of which are connected to spaced, outwardly directed support structures called crank arms. The main bearing journals are normally provided at spaced intervals along the length of the entire crankshaft between adjacent pairs of crank arms. The connecting rod journals, or crank pins, are attached to the crankshaft between the opposite ends of the crank arms, also at spaced intervals. As the name suggests, the connecting rod journals provide the bearing surface for the connecting rod associated with an adjacent piston (or pistons, in the case where two pistons are connected to the same connecting rod journal). In the usual arrangement, the two types of journals are eccentric to each other. Hence, as the pistons move to and fro, the connecting rods either push or pull on the connecting rod journals and cause the crankshaft to rotate 360 degrees.
Despite the popularity of this arrangement, one significant downside is that, because the main bearing journals are discontinuous (that is, provided between the crank arms at spaced intervals), the crankshaft is susceptible to twisting and bending (possibly as much as 16° when subjected to high loads). This bending is troublesome, since it not only reduces efficiency and creates timing issues, but may also cause cracks to form at the interface between the crank arms and the journals that can ultimately lead to mechanical failure of the crankshaft. Another problem with this arrangement is that the crankshaft is subjected to significant vibrations because of the presence of the connecting rods simultaneously pushing and pulling on each individual connecting rod journal. Due to these inherent limitations, crankshafts constructed in the foregoing manner are somewhat limited both in service life and in the amount of power that can be transmitted.
Recent designs have attempted to address these problems, such as by adding support structures or counterweights to the crankshaft or attempting to make it more rigid. Although these measures have extended the service life of the crankshaft, increased the amount of power that can be transmitted, and reduced vibrations, other problems result. For example, a consequence of adding support structures is that the crankshaft is made larger, which necessarily increases the size of the engine in which it is used. Additionally, the more complicated the design, the more the manufacturing cost of the crankshaft increases.
Thus, a need is identified for a crankshaft that provides all the advantages of the prior art designs, but eliminates the many disadvantages. The crankshaft would be significantly stronger and better able to resist bending without a corresponding increase in size or a significant increase in weight. The connecting rods required in most prior art proposals would be eliminated, which reduces the complexity of the design and decreases the amount of vibrations experienced by the crankshaft during operation. Overall, the arrangement provided would be a significant improvement over prior art proposals, especially in terms of ease of manufacture and reliability.